The Effect of Shade and Thickness on the Depth of Cure of Bulk-Fill Composites with Different Viscosities

Statement of the Problem: In an attempt to enhance and simplify the restoration process, a new class of composite resins, called the bulk fill composite resins have been introduced. It is claimed that a depth of cure (DOC) of 4 mm can be achieved without affecting the properties of this material. Purpose: The purpose of this study was to investigate the effect of different shades, thicknesses, and viscosities on the DOC of bulk-fill composites. Materials and Method: In this experimental study, four bulk-fill composites [Filtek™ Bulk Fill Flowable (FBF), Filtek™ Bulk Fill posterior (FBP), Tetric® N-Flow Bulk Fill (TNF), Tetric® N-Ceram Bulk Fill (TNC)] and a conventional composite, Filtek™ Z250 XT Universal (FZ) were evaluated. The samples (n=5) were made using two different shades (light and dark), thicknesses (2 and 4mm), and viscosities (flowable and sculptable). Microhardness test was conducted on top and bottom surface using Vickers microhardness tester and DOC was calculated as the bottom/top ratio of yielded scores. Statistical analysis was done using a Mann Whitney test at p< 0.05. Results: DOC ranged between 52-95%. FBF composite exhibited the lowest overall hardness numbers. At 2-mm thickness, all the samples achieved an appropriate DOC. However, at 4mm thickness, only the light shades for FBF and TNF samples achieved a DOC very close to 0.8. At 4-mm thickness, the light shades for FBF, TNF and FZ samples exhibited significantly higher DOC compared to dark shades. For 4-mm-thick samples, the DOC of Filtek™ Bulk Fill (dark and light shades (and the DOC of Tetric® Bulk Fill (light shade (were different in flowable type from the sculptable type. Conclusion: The shade and the viscosity of bulk-fill composites influence their DOC at 4-mm depths. Moreover, 20 seconds of light curing appears insufficient for 4mm thickness of bulk-fill composite.


Introduction
Currently, direct composite resins are the preferred materials for restoring small to medium cavities in posterior teeth on conditions in which the bonding and filling procedures can be properly performed [1]. Conventionally, to restore cavities with incremental technique, the composite resin is cured at a maximum thickness of 2mm. The main advantage of this technique is optimal cure throughout the material depth and decreased polymerization shrinkage [2]. On the other hand, the incremental technique is time-consuming, with higher risk of air bubbles being trapped between the layers and contamination of the operating field due to increased working time [3].
Recent developments in the technology of composite resin production have led to the introduction of bulk-fill composite resins, which can be cured in a thickness of 4-5 mm, resulting in a decrease in the duration of the restorative procedure [4]. Various studies have evaluated the physical properties of bulk-fill composites resins, including creep [5], modulus of elasticity [6], cuspal deflection [7], microleakage [8], and wear resistance [9].
As a material classification, the assessed mechanical properties put the bulk-fill composite resins between the nanohybrid and microhybrid composite resins and the flowable composite resins, signifying a parallel or even lower clinical performance of bulk-fill composite resins compared to nanohybrid and microhybrid composite resins [4]. Bulk-fill composite resins are also comparable with conventional composite resins considering water uptake and biocompatibility [10].
One of the most important factors in the failure of composite resin restorations is inadequate curing. Uncured composite resins might result in the failure of restoration because of increased chance of fracture, recurrence of caries or wear of the restoration. On the other hand, when the composite resin is not adequately cured, there is an increased risk of leakage of chemical materials from composite resin into the body tissues [11]. According to previous studies, the type of composite resin photoinitiator, filler type, matrix, color, translucency, light spectrum of the light-curing unit, and composite placement technique affect the depth of cure (DOC) of composite resins [12]. In addition, the thickness of the composite resin, irradiation time and the intensity of light influence the degree of conversion [13].
Typically, there are some methods for evaluating the adequate curing for a resin including degree of conversion using Fourier-transform infrared (FTIR) spectroscopy, and microhardness test. The majority of studies have indicated a good correlation between the degree of conversion and the microhardness test [14][15][16]. In the microhardness test, optimal DOC is defined as a depth with a hardness ratio of at least 0.8 of the hardness of composite resin surface [15][16][17]. Some researchers recommend the cure of bulk-fill composite resins at a thickness of 4 mm [18][19]; others believe that the methods used in reported studies have overestimated DOC of these composite resins and proclaim that the polymerization of bulk-fill composite resins at 4-mm depths is inadequate [11,20].
Bulk-fill composite resins are divided into two groups based on viscosity: bulk-fill composite resins with low viscosity (flowable) and bulk-fill composite resins with high viscosity (sculptable).
The aim of this study is to evaluate the effect of viscosity, shade, and thickness on the DOC of bulk-fill composite resins. The null hypothesis states that DOC of the evaluated composite resins is not affected by viscosity, shade, and thickness.

Materials and Method
In this experimental study, four types of bulk-fill com-  Table 1. The samples were assigned to 20 groups and five samples were prepared for each group [21]. Steel molds, measuring 4 mm in diameter and 2 or 4 mm in thickness were used to prepare the samples [22]. After placing the mold on a glass slab and celluloid matrix strip, the composite resin was packed within it; then a glass slab and a   Kolmogorov-Smirnov test was used to evaluate the normality of data. Since data were not normal, Mann-Whitney test was used to compare each variable individually. SPSS 24 was used for statistical analysis at a significance level of P<0.05. Table 2 presents the mean hardness of the top and the bottom for each sample in terms of shade, viscosity and thickness.   resins' DC and microhardness [27]. Therefore, we concurrently evaluated the effect of viscosity, shade, and thickness on the DOC of bulk-fill composite resins.

Results
Consistent with the results of previous studies [19][20], hardness of the top was higher than that of the bottom. FZ composite resin exhibited the highest surface hardness, followed by FBP composite resin. In line with the results of previous studies [20,28], in the current study, the surface hardness in the flowable type was significantly lower than that in the sculptable type in each composite resin, which could be due to the lower filler content of flowable composite resins. Studies have shown that the filler content of composite resins could affect their hardness and physico-mechanical properties [29]. The results of this study showed lower DOC in all the composite resins in 4-mm-thick samples compared to 2-mm-thick samples (Table 2), consistent with the results of previous studies [19][20]. A likely rationale could be the absence of light penetration through the composite at increasing depths since a high percentage of the wavelengths are absorbed in approximate to the top surface of the composite, subsequently it cannot excite co-initiators at larger depths [30].
Generally, manufacturers use methods such as in creasing translucency, increasing the amount of photo initiators, and use of additional photo initiators to increase DOC of composite resins [31]. In this context, these composite resins have less light attenuation and more light transition rates compared to conventional co-mposite resins [20].
In agreement with the results of previous studies  [11].
The impact of mold size has been studied for opaque cylindrical molds and the results showed that DOC would decrease if the mold size diameter were decreased [36]. Black molds presented shorter DOC than a stainless steel mold once a light shade of composite was cured [37]. The results of this study indicated no significant differences in the curing depths between the dark and light shades except between two flowable bulk-fill composite resins with 4-mm thickness. However, Rodriguez et al. [19] concluded that when Tetric EvoCeram Bulk Fill and SonicFill™ composite resin samples with 4-mm thickness were light-cured for 20 seconds, there was a significant difference in curing depth between dark and light shades and DOC in light shades was greater than that in dark shades [40].
The size, radioactivity, translucency, and pigments in the filler particles affect light transmission of the material [41]. Pigments in dark shades limit the light transmission and reduce the degree of polymerization [30]. It seems that due to lower filler content and higher translucency of flowable bulk-fill composite resins compared to sculptable type, presence of more pigments in the dark shade resulted in a decrease in curing depth when the thickness of composite has been increased resins to 4 mm [42].  [3]. This variation in the DOC of bulk-fill composite resins with different viscosities might be related to the difference in their filler content. By increasing filler-to-matrix ratio, the degree of conversion decreases since high filler content prevents the development of polymer chains [43]. In addition, as the amount of filler increases, the amount of light scattering increases and the translucency for the blue color decreases [26].
Investigating by scanning electron microscope, bulk-fill flowable showed large filler size with dominant polygonally shaped characteristics compared to conventional flowable resin composites. The filler load was slightly increased, however, because of the bigger size of the filler particle, the filler-matrix interface was supposed to be decreased. Therefore, it permits more curing light to transmit through the composite and improve the DOC [44].
In this study only four types of bulk-fill composite resins were studied; therefore, it is suggested that other bulk-fill composite resins should be studied and the effects of other variables, including the intensity of radiation, type of light-curing unit and its distance from the surface of composite resin, on their DOC could be investigated in future studies.

Conclusion
The